1. Field of the Invention
The present invention relates to a digital signal processing technique. Specifically, the present invention relates to an audio processing unit for realizing high sound quality reproduction by decoding an audio compress-encoded signal and conducting a band extension process.
2. Description of the Related Art
Various methods have been proposed for realizing high-sound-quality reproduction by conducting a band extension process. In one method, a band is extended to realize a reproduction with high sound quality by conducting a N times oversampling (N is an integer bigger than 1) a decoded PCM signal having a sampling frequency Fs, and adding to the oversampled PCM signal having a sampling frequency of N×Fs/2 a noise signal having a band component of a band from Fs/2 to N×Fs/2. This method is performed, for example, using an audio processing unit having a structure as shown in FIG. 25.
This audio processing unit 10000 is composed of a decode-processing section 1100, an oversampling section 1200, an extended band generating section 1300 and a bandpass filter 1400.
The decode-processing section 1100 has a function of decoding an encoded audio stream inputted from outside and generating PCM data. The oversampling section 1200 receives the PCM data decoded by the decode-processing section 1100 so as to perform a N times oversampling, and outputs PCM data that is oversampled N times. The extended band generating section 1300 has a function of generating a band extension component with respect to the PCM data oversampled by the oversampling section 1200. The bandpass filter 1400 is a filter for passing components that are in a band from around Fs/2 to N×Fs/2 among band extension components generated by the extended band generating section 1300.
For facilitating the explanation, the decode-processing section 1100 is regarded here as a decoder corresponding to a DVD-Video standard, and this description refers to a case where an audio bit stream according to a DVD-Video standard linear PCM is inputted. Here, the inputted audio bit stream has a sampling frequency of 48 kHz. The oversampling section 1200 generates PCM data having a sampling frequency of 96 kHz by inserting one sample of ‘0’ data to each spacing between the PCM data having a sampling frequency of 48 kHz outputted from the decode-processing section 1100, conducting the oversampling, and rejecting a noise by use of an antialiasing filter. The extended band generating section 1300 has a function of generating a harmonic bandpass extension component of at most approximately 48 kHz on the basis of the PCM data of 96 kHz oversampled by the oversampling section 1200. The bandpass filter 1400 passes components that are in a band from about 24 kHz to about 48 kHz among band extension components generated by the extended band generating section 1300.
As shown in FIG. 25, when an audio bit stream according to a linear PCM of a DVD-Video standard is inputted to the decode-processing section 1100, the decode-processing section 1100 generates PCM data having a sampling frequency, a channel mode, a quantization bit length based on an encoding mode represented at a private head, and outputs the data. The PCM data generated here is a signal that can have a band property of at most 24 kHz due to a Nyquist condition, since the data has a sampling frequency of 48 kHz, and the input bit stream data is an uncompressed and reversible encoded signal.
Next, the oversampling section 1200 inserts one sample of ‘0’ data between the respective PCM data generated at the decode-processing section 1100, and transforms it into PCM data having a sampling frequency of 96 kHz by using an antialiasing filter. At this time, the band property of the PCM data is at most 24 kHz just as the input PCM data, since an antialiasing noise is reduced by using the antialiasing filter.
The extended band generating section 1300 generates a band extension component formed of a harmonic of at most about 48 kHz based on PCM data having sampling frequency of 96 kHz processed at the oversampling section 1200, and outputs the component. The thus generated band extension component has an oversampling frequency of 96 kHz, the same as the PCM data processed at the oversampling section 1200. The band extension component generated at the extended band generating section 1300 is limited to a band from about 24 kHz to a band lower than about 48 kHz by a bandpass filter 1400, and the output data is added to PCM data processed at the oversampling section 1200, and outputted to the outside.
In such a case of an audio processing unit, when inputted encoded data is an uncompressed and reversible encoded audio signal such as an audio stream according to a DVD-Video standard linear PCM, it can be designed as a band extension component under a Nyquist condition, and thus, a targeted band extension region can be set fixedly as described above. Accordingly, an excellent band extension effect can be expected.
However, in a case of an audio processing unit targeting a DVD standard audio encoded signal, which has been spread recently in the market, an encoded signal to be decoded is not limited to linear PCM, but encoded signals with information compression should be considered as well, such as Dolby digital encoding system (AC3) and a MPEG audio standard. These encoding systems depend on irreversibly-encoded signals subjected to information compression on the basis of human audibility, a masking effect or the like. In many cases, linear PCM with a sound source having a sampling frequency of 48 kHz before encoding is compressed to 10-20% after an encoding. Even when a band component is contained in a band of about 24 kHz under a Nyquist condition (Fs/2) in a sound source before encoding, the component will be dropped at the time of encoding.
In many cases of inputting such an encoded signal, PCM data decoded by the decode-processing section 1100 contains substantially no band components around the Nyquist condition in the above-mentioned audio processing unit. Therefore, even by conducting a band extension process as mentioned above, a linear band extension will not be performed on a frequency axis. Since this leads to a dropout of a band component in a range from an upper limit of the band of PCM data decoded by the decode-processing section 1100 to around the Nyquist frequency (Fs/2), a sufficient band extension effect cannot be obtained.